The cost of supplying electrical energy includes fixed costs which depend upon the size of the installation and variable operating costs which are proportional to the output thereof. In order to distribute these costs most effectively to the consumers, electric power companies operate in accordance with a maximum tariff. Thus, apart from the actual consumption charge, there is applied a fee, generally known as the demand charge, which is determined by the highest power actually supplied over a predetermined period.
The measurement of electric power demand is carried on by a maximum or peak indicator or a demand meter.
For calculating the demand charge, it is desirable that brief or intermittent power peaks not be considered since they do not generally affect the fixed costs of generating electric power. As a result, it is desirable to use the value of the mean power over a relatively long period of 15 to 30 minutes for determining the power maximum. Conventional maximum indicators have a circular scale provided with a drag indicator. At the end of the desired reading time, the position of the drag indicator or pointer against the scale indicates the highest power maximum which has been detected.
Naturally, each consumer of power would like to hold its power maximum (demand) as low as possible. To this end, monitoring devices have been provided so that, when the maximum indicator reaches a predetermined power level, various loads or groups of loads are cut off from the supply network. Such automatic load-shedding reduces the drain and hence the power consumption of the monitored system and thereby prevents an increase in the demand charge.
The disadvantage of conventional monitoring devices is that they generally must be coupled with the maximum-power indicator and hence must be connected by control lines with the loads or groups of loads to be cut off. In a large system, e.g. a large industrial plant, farm or the like, these control lines may run distances of the order of kilometers. Another important disadvantage is to be found in the nature of the monitoring process. Conventional devices only become effective when a predetermined power level is attained to operate the load-shedding switches. This power level thus constitutes a switching threshold.
However, when the rate of increase of the power consumption is sharp, the actual triggering of the load-shedding devices may be too late to prevent an increase in the demand charge. On the other hand, the need to respond quickly once the threshold is reached may result in the cut off of a greater number of loads than is desirable so as to interrupt operation of the factory or otherwise interfere with the normal operating procedures of the system. Frequently, therefore, the use of the prior art system is a compromise between premature and undesirable cut off loads and delay in effecting the cut off of loads so that the demand charge will increase.